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May 18, 2007

Early detection of soybean rust poses threat to Hoosier crop

WEST LAFAYETTE, Ind. - Indiana is at greater risk for developing soybean rust this year than in the past two years, said a Purdue University expert.

Plant pathologists reported finding soybean rust 53 days earlier this year than last on kudzu in southern Louisiana.

"This is not good news for Indiana soybean growers because it opens the window of time for rust to develop in the Hoosier state," said Greg Shaner, Purdue Extension soybean rust specialist.

Soybean rust first appeared in Indiana last year, but it arrived in October - too late to damage the crop, Shaner said. If the rust had arrived earlier in the growing season, as it threatens to do this year, it could have resulted in yield losses.

It is still too early to predict the likelihood and potential intensity of soybean rust in Indiana this year, but the early breakout has researchers on the lookout. Timing is important because the soybean plant is most vulnerable in the flowering stage, which occurs in late June and early July.

"To the best of our knowledge all of our soybean varieties are susceptible to rust, and the only way to control it is to use a fungicide," Shaner said.

If soybean rust is found in Indiana, all the agricultural and natural resources Extension educators will be notified, and efforts will be made to get information to growers.

One thing growers can do to prepare is to check with their local chemical dealer and make sure a fungicide will be available in late July or early August, if needed. However, Shaner does not believe there is a need to stockpile fungicides.

Soybean rust overwinters in Florida and along the Gulf Coast, and the spores migrate north as the weather warms, drifting on the wind like smoke from a smokestack.

"Given the right weather conditions, soybean rust can travel up to 500 miles in two or three days," Shaner said.

Rainfall scrubs spores from the air and deposits them on the plants below. Depending on rainfall intensity, some spores may escape being washed down and will continue migrating, so deposition can occur over a wide area.

To help determine the path of these spores, scientists look at the wind trajectory and where rain falls, which helps determine where spores will be deposited. Last week's heavy rains in Louisiana created ideal conditions for the development of rust, and more rain is expected, Shaner said. However, the rust in Louisiana does not pose an immediate threat to Indiana because soybeans are just being planted here.

If it were fairly dry immediately north of a source of rust spores in Louisiana, there may be a gap in the rust outbreak. The disease would leapfrog, perhaps skipping 100 or 200 miles and appear in Arkansas or Tennessee. If the scrubbing wasn't thorough enough over that area, some spores could keep moving north, all the way to Indiana.

Indiana's summer weather is probably favorable for the development of soybean rust in most years. Once the spores land on a leaf, they need moist conditions for at least seven hours or more to germinate.

Soybean rust can move fast - it only takes nine days from the time a spore infects for that infection to start producing pustules. Initial infections that develop from spores coming from the south will most likely be few, and they are hard to detect at this stage, Shaner said. But, the very early stages of disease development, when no more than 10 percent of leaves show any infection, is the threshold for effective disease control with a fungicide.

During the summer months, scientists will be out collecting 100 leaves from each of Indiana's 20 sentinel plots each week, Shaner said. The leaves will be shipped overnight to a Purdue lab and examined by scientists for signs of rust. Each 50-by-50 square-foot plot is just a small fraction of the total soybeans planted in the state, but the intensive scouting of these plots, along with examination of similar plots in all states south of Indiana, provides an effective early warning system for the disease, Shaner said.

In addition to monitoring the soybean sentinel plots, several kudzu sites in the state will be monitored.

Writer: Julie Douglas, (765) 496-1050, douglajk@purdue.edu

Source: Greg Shaner, (765) 494-4651, shanerg@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
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